Problem: Let $x,$ $y,$ $z$ be positive real numbers such that $x + y + z = 1.$  Find the minimum value of
\[\frac{1}{x + y} + \frac{1}{x + z} + \frac{1}{y + z}.\]
Explanation: By Cauchy-Schwarz,
\[[(x + y) + (x + z) + (y + z)] \left( \frac{1}{x + y} + \frac{1}{x + z} + \frac{1}{y + z} \right) \ge (1 + 1 + 1)^2 = 9,\]so
\[\frac{1}{x + y} + \frac{1}{x + z} + \frac{1}{y + z} \ge \frac{9}{2(x + y + z)} = \frac{9}{2}.\]Equality occurs when $x = y = z = \frac{1}{3},$ so the minimum value is $\boxed{\frac{9}{2}}.$